Mineral oil lubricants are derived from various crude oil stocks by a variety of refining processes. Generally, these refining processes are directed towards obtaining a lubricant base stock of suitable boiling point, viscosity, viscosity index (VI) and other characteristics. Generally, the base stock will be produced from the crude oil by distillation of the crude in atmospheric and vacuum distillation towers, followed by the separation of undesirable aromatic components and finally, by dewaxing and various finishing steps. Because aromatic components lead to high viscosity and extremely poor viscosity indices, the use of asphaltic type crudes is not preferred as the yield of acceptable lube stocks will be extremely low after the large quantities of aromatic components contained in such crudes have been separated out; paraffinic and naphthenic crude stocks will therefore be preferred but aromatic separation procedures will still be necessary in order to remove undesirable aromatic components. In the case of the lubricant distillate fractions, generally referred to as the neutrals, e.g. heavy neutral, light neutral, etc., the aromatics will be extracted by solvent extraction using a solvent such as phenol, furfural, N-methylpryyolidone (NMP) or another material which is selective for the extraction of the aromatic components. If the lube stock is a residual lube stock, the asphaltenes will first be removed in a deasphalting step e.g. by propane deasphalting, followed by solvent extraction of residual aromatics to produce a lube generally referred to as bright stock. In either case, however, a dewaxing step is normally necessary in order for the lubricant to have a satisfactory low pour point and cloud point, so that it will not solidify or precipitate the less soluble paraffinic components under the influence of low temperatures.
A number of dewaxing processes are known in the petroleum refining industry and of these, solvent dewaxing with solvents such as methylethylketone (MEK) and liquid propane, has been the one which has achieved the widest use in the industry. Recently, however, proposals have been made for using catalytic dewaxing processes for the production of lubricating oil stocks and these processes possess a number of advantages over the conventional solvent dewaxing procedures. The catalytic dewaxing processes which have been proposed are generally similar to those which have been proposed for dewaxing the middle distillate fractions such as heating oils, jet fuels and kerosenes, of which a number have been disclosed in the literature, for example, in Oil and Gas Journal, January 6, 1975, pp. 69-73 and U.S. Pat. Nos. RE 28,398, 3,956,102 and 4,100,056. Generally, these processes operate by selectively craking the longer chain end paraffins to produce lower molecular weight products which may then be removed by distillation from the higher boiling lube stock. The catalyst which have been proposed for this purpose have usually been zeolites which have a pore size which admits the straight chain, waxy n-paraffins either alone or with only slightly branched chain paraffins but which exclude more highly branched materials and cycloaliphatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35 and ZSM-38 have been proposed for this purpose in dewaxing processes, as described in U.S. Pat. Nos. 3,894,938, 4,176,050, 4,181,598, 4,222,855, 4,229,282 and 4,247,388. A dewaxing process employing synthetic offretite is described in U.S. Pat. No. 4,259,174. The Mobile Lube Dewaxing process (MLDW) is capable of converting high pour point waxy lube oil base stocks by selective cracking over a shape selective catalyst, followed by hydrotreating to saturate lube range olefins so as to stabilize the product. See 1986 Refining Process Handbook (Hydrocarbon Processing, September 1986, Gulf Publ. Co.) page 90 and Catal. Rev. - Sci. Eng. 28 (283), 185-264 (Chen et al), especially 244-247.
Although the catalytic dewaxing processes are commercially attractive because they do not produce quantities of solid paraffin wax which presently is regarded as an undesirable, low value product, they do have certain disadvantages and because of this, certain proposals have been made for combining the catalytic dewaxing processes with other processes in order to produce lube stocks of satisfactory properties. For example, U.S. Pat. No. 4,181,598 discloses a method for producing a high quality lube base stock by subjecting a waxy fraction to solvent refining, followed by catalytic dewaxing over ZSM-5 with subsequent hydrotreatment of the product. U.S. Pat. No. 4,428,819 discloses a process for improving the quality of catalytically dewaxing lube stocks by subjecting the catalytically dewaxed oil to a hydroisomerization process which removes residual quantities of petrolatum wax which contribute to poor performance in the Overnight Cloud Point test (ASTM D2500-66). This process is intended to overcome one disadvantage of the intermediate pore dewaxing catalyst such as ZSM-5 which is that the normal paraffins are cracked much faster that the slightly branched chain paraffins and cycloparaffins so that, although a satisfactory pour point is attained (because the straight chain paraffins are removed) residual quantities of branched chain paraffins and cycloparaffins may be left in the oil, to contribute to a pour performance in the Overnight Cloud Point test when the oil is subjected to a relatively low temperature for an extended period of time. During this time, the petrolatum wax which is made up of the less soluble slightly branched chain paraffins and cycloparaffins, nucleates and grows into wax cyrstals of a sufficient size to produce a perceptible haze. Although it would be possible to remove the petrolatum wax by operating the dewaxing process at a higher conversion so that these components were removed together with the straight chain paraffins, the yield loss which would result, has generally been considered unacceptable. Thus, the need for a subsequent processing step has become apparent.
As mentioned above, the conventional catalytic dewaxing processes using intermediate pore size zeolites such as ZSM-5 operate by selectively cracking the waxy components of the feed. This results in a loss in yield since the components which are in the desired boiling range undergo a bulk conversion to lower boiling fractions which, although they may be useful in other products, must be removed from the lube stock. A notable advance in the processing of lube stocks is described in U.S. Pat. Nos. 4,419,220 and 4,518,485, in which the waxy components of the feed, comprising straight chain and slightly branched chain paraffins, are removed by isomerization over a catalyst based on zeolite beta.
During the isomerization, the waxy components are converted to relatively less waxy isoparaffins and at the same time, the slightly branched chain paraffins undergo isomerization to more highly branched aliphatics. A measure of cracking does take place during the operation so that not only is the pour reduced by reason of the isomerization but, in addition, the heavy ends undergo some cracking or hydrocracking to form liquid range materials which contribute to a low viscosity product. The degree of cracking is, however, limited so as to maintain as much of the feedstock as possible in the desired boiling range. As mentioned above, this process uses a catalyst which is based on zeolite beta, together with a suitable hydrogenation-dehydrogenation component which is typically a base metal or a nobel metal, usually of group VIA or VIIIA of the Periodic Table of the Elements (the periodic table used in this specification is the table approved by IUPAC), such as cobalt, molybdenum, nickel, tungsten, palladium or platinum. As described in U.S. Pat. No. 4,518,485, the isomerization dewaxing step may be proceded by a hydrotreating step in order to remove heteroatom-containing impurities, which may be separated in an interstage separation process similar to that employed in two-stage hydrotreating-hydrocracking processes.
As is apparent from the preceding description, the objective in dewaxing processes is to remove the waxy components of the feed which tend to precipitate out of the liquid oil when it is subjected to low temperatures. These waxy components may generally be characterized as the straight chain and slightly branched chain paraffins of high melting point, especially the mono-methyl paraffins. Generally, the straight chain paraffins must be removed in order to ensure that the oil has a satisfactory low pour point while the slightly branched chain materials need to be removed in order to ensure that the product does not become hazy by the relatively slow growth of the waxy components. If especially low pour points are desired, it may be necessary to remove some of the higher melting point branched chain paraffins such as the mono-methyl paraffins because preferential removal of the n-paraffins will generally lower the pour point at about -18.degree. C. (-28.degree. F.). A countervailing factor, however, is that it is generally undesirable to operate the dewaxing under conditions of relatively high severity because not only does this result in a lower lube yield but, in addition, the isoparaffinic components which contribute to a high viscosity index may be removed together with the waxy components which are more straight chain in character. Thus, a balance must be sought between removing sufficient of the waxy paraffins to obtain the desired pour point and cloud point specifications and the need to retain a sufficient number of the branched chain isoparaffins which contribute to a good viscosity index (VI) in the product. It is, of course, desirable to produce a base stock of high VI since this reduces the need for VI improvers which, besides being expensive, become degraded in use with a resultant deterioration in lubricant properties. The objective of the dewaxing procedure must therefore be to produce a lube stock with an acceptable balance of properties in as high a yield as possible.